lguapo, you have not said anything about the accessibility of this rotted piece of plywood. If it can come out in one piece, the situation would be pretty simple, if not, pretty complicated.
In any case, the fact that it rotted (no surprise in an anchor locker) may well suggest that you replace it with one of the plastic boards rather than wood
. If the existing plywood has to come out in two pieces, use them as patterns, install, and then splice the two together with a strip of board along the seam.